Please explain the problem in detail. What happens? Error messages?

Make sure you close Firefox properly and use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

You can check the startup setting.

• Options/Preferences -> General -> Startup: "When Firefox starts":
  "Show my windows and tabs from last time"
  "Show my home page"
  "Show a blank page"